I read once that it was possible to 'steal' a normal layer with a mask by using Photomerge, without any add-on nor any use of clipping layer groups. (I am using PSE6)

Here is the workflow I found:- Start with File/new/Photomerge panorama- in the dialog box, chose from Files and browse- select 2 random pictures, click OK and let the script go. Ignore the warning.

Have a look at the layers palette: you have two layers with a black mask and a white mask

Now, imagine you want to add a mask to a standard layer.

The idea is:1- to move one of those layers under the layer lacking a mask. To do so, I suppose you have both the photomerge and your image open side by side (Window/image/tile).The photomerge image being active, click on one of the layers in the layer palette and drag it on your own image.

2- Select the copied layer in your own image, select all (Ctrl A) and delete to have a blank layer with a mask. Deselect (Ctrl D).

3- Instead of copying the mask, what you'll do now is to insert the content of the layer lacking a mask in the blank layer with a mask. Make your own layer without a mask active, and select all (Ctrl A), copy (Ctrl C).now, make the photomerge layer active, and use Edit/Paste into Selection (shift Ctrl V) to paste the content into the photomerge layer. You can now deactivate or delete the original layer without a mask.

I do not pretend this is the best way to add a layer mask, but it is a proof that the capacity of normal layers is at hand and wickedly hidden by Adobe.